The present invention relates to a method for assessing (analyzing) the risk of potential adverse effects for a human patient mediated by the administration of a CD19xCD3 bispecific antibody to said patient comprising determining the ratio of B cells to T cells of said patient, wherein a ratio of about 1 :5 or lower is indicative for a risk of potential adverse effects for said patient or determining the total B cell count of said patient, wherein a total B cell count of less than about 50 B cells per microliter of peripheral blood is indicative for a risk of potential adverse effects for said patient. Accordingly, the present invention relates a method (dosage regimen) for administering a CD19xCD3 bispecific antibody to a human patient having a B:T cell ratio of about 1 :5 or lower and/or a total B cell count of less than about 50 B cells per microliter of peripheral blood, comprising (a) administering a first dose of said antibody for a first period of time； and consecutively (b) administering a second dose of said antibody for a second period of time, wherein said second dose exceeds said first dose. In some embodiments, a third dose of said antibody is administered for a third period of time. This dosage regimen can be applied in methods for treating malignant CD19 positive lymphocytes or for ameliorating and/or preventing an adverse effect mediated by the administration of said bispecific antibody. The present invention also relates to the use of a CD19xCD3 bispecific antibody for the preparation of a pharmaceutical composition to be used in a method of the present invention. A pharmaceutical package or kit comprising a first dose and a second dose and optionally a third dose of said antibody as defined in the methods/dosage regimen of the present invention is disclosed as well.La présente invention concerne un procédé d'évaluation (d'analyse) du risque d'effets indésirables potentiels pour un patient humain médiés par l'administration d'un anticorps bispécifiq
